You wrote several ideas that I was vaguely aware of, but had not put into words for myself. All your points were simply common sense, in my opinion; thank you for posting.

I do kind of think, however, that where Sir Thomas is coming from is that he never even considered that a man of means, like Henry Crawford, would be interested in Fanny, of all people. I think Sir Thomas considers her to be like a mouse: something to be politely ignored, or gotten rid of.